To investigate the possibility of phylogenetic relatedness between the vertebrate major histocompatibility complex and the gene systems controlling allogeneic tissue reactions in protochordates, we propose to characterize the mechanisms and molecular structures involved in the colony specificity phenomenon in tunicates of the genus Botryllus. By use of adoptive transfer systems, we hope to determine whether allogeneic recognition and effector responses in this system are controlled by blood cells or by humoral hemolymph components. By this approach, we hope to show that both the afferent and effector phases of these responses are controlled by cell surface recognition components. If this is the case, we should be able to develop bioassays for cell surface moieties relevant to recognition events and effector responses in this system by developing conditions in which antisera raised against cell surface moieties on Botryllus cells can inhibit colony fusion or rejection reactions. If inhibiting antisera can be developed in mice, we will use a recently developed technique to produce pure antibodies to cell surface components. B cell clones from spleens of immunized mice will be established in culture as cell lines producing antibody against single determinants. Culture fluids from groups of clones will be screened for inhibiting activity, and active antisera will be used in immunoprecipitation procedures for preparation of specific components from tissue cells. By physicochemical characterization, peptide analysis, and limited amino acid sequence analysis, these proteins will be examined for structural and amino acid sequence homology to gene products of the vertebrate major histocompatibility complex and related systems.